1. Field of the Invention
The present invention relates to a control system suitable for use in an air-compressing apparatus including, for example a plurality of compressors that individually supply compressed air to a tank.
2. Description of Related Art
A generally known type of air-compressing apparatus has a plurality of compressors connected in parallel to a tank (for example, see Japanese Patent Application Publication Nos. 2003-21072 and 2003-35273). The air-compressing apparatus is provided with a pressure sensor to measure the pressure in the tank. The pressure value detected with the pressure sensor is compared with a plurality of predetermined control threshold values to perform loading/unloading control and start/stop control of each compressor. With this system, the number of compressors to be operated is varied according to the pressure in the tank to adjust the discharge flow rate of compressed air supplied to the tank.
The above-described prior art controls the number of compressors to be operated by comparing the detected value of the pressure in the tank with the control pressure values. Therefore, there are cases where an unnecessarily large number of compressors are operated because the detected tank pressure has become lower than a predetermined control threshold value despite a very low consumption of compressed air from the tank. There are also cases where a plurality of compressors are driven until the pressure in the tank reaches a maximum pressure irrespective of the consumption of compressed air. In such cases, the electric power is consumed wastefully.
It is also conceivable to provide a flow sensor in the output piping of the tank and to detect the consumption of compressed air with the flow sensor, thereby performing the compressor control according to the consumption of compressed air. In this case, however, an extra flow sensor needs to be provided, and the installation of the flow sensor increases the number of man-hours. Therefore, the production cost increases unfavorably.